To an ever increasing extent, magnet coil configurations of the above mentioned kind are used in the field of nuclear spin resonance (NMR) technology for the production of magnetic fields or magnetic gradient fields. However, they may also be used in other technological fields. One of the main requirements is that, with the magnet coil configuration, a predetermined magnetic field distribution has to be produced as exactly as possible and the inductance L of the magnet coils must be as small as possible. Other boundary conditions must also be observed such as, for example, an optimum shielding of the exterior from the magnetic field produced inside the magnet coil configuration and minimized oscillation of the current density distribution produced by the current carrying magnet coils.
In the above mentioned publication, EP-A1 0320 285, an algorithm is described to calculate the geometrical data of such a magnet coil configuration which allows the calculation of the current density of the main and shielding coil in such a way that, with perfect shielding, the inductance L is minimum and the desired distribution of the magnetic field is given as defined only in an indistinct way by fixing singular field points. This algorithm is not limited to gradient systems, but may in principle be applied to all coil systems. Aside from the field points fixed at the outset, the resulting field produced by the magnet coil system and the corresponding field errors can be calculated at all points only after calculation of the current densities.
Since merely singular field points are predetermined by this procedure, the corresponding global field error is by no means defined a priori. The magnetic field may oscillate between the singular field points. A definition of the desired magnetic field including defined permissible field errors at the beginning would only be possible by predetermining the global field or by an intelligent, specific system of selecting geometrically well defined field points, whose relative geometry had to be fitted to the individual case. However, such a selection system is not described in the above mentioned publication EP-A1 0320 285. According to the specification, the disclosed method may only be described as a "trial and error", method with respect to the desired magnetic field distribution and defined maximum permissible errors.
As a consequence of the described "one way" algorithm, the known method yields no direct possibility to influence the variation of the current density distribution. In order to counteract unacceptably strong oscillations of the current density distribution and multiple current inversion, i. e. multiple change of sign of the winding directions within the coil, the known method suggests using an "apodising function". However, this careful smoothing works only outside the algorithm for minimum inductance etc., quoted in the publication. As is the case with the above mentioned field error, a technically realizeable current density distribution can also only be found by "trial and error", with this method. However, the above mentioned publication does not explicitly describe such a method or a corresponding set of parameters for smoothed shielding coils with minimum inductance. On the other hand, each current density distribution which deviates from the oscillating theoretically derived result additionally has the retroactive consequence of an only insufficient realization of the field distribution of the predetermined intended field, shielding effect and minimum inductance.
From all this, it is clear that the known method lacks any possibility of a direct coupling of global permissible field errors to the remaining properties of the magnet coil configuration which has to be calculated. Therefore there is also no practical possibility to optimize each desired magnet coil system to the one configuration with minimum errors. Moreover, the strongly oscillating current density distribution as calculated according to the known method can either not at all be technically realized or only in a limited sense. As a consequence, however, the properties coupled mathematically to the respective current density distribution (with or without oscillations) as minimum inductance, maximum shielding, etc. can also only be realized in a limited sense.
It is therefore the object of the invention to present a method of the above mentioned kind which yields results that can technically be used as easily as possible whereby in addition to the required minimization of the inductance L within predetermined boundary conditions, also additional technically relevant parameters of the magnet coil configuration can be optimized independently from each other.